Method for controllable synthesis of carbon based battery electrode material

ABSTRACT

Carbon-based electrode materials including graphite particles bridged by hemispheres of fullerene, as well as methods of synthesizing the carbon-based electrode materials, are disclosed. These carbon-based electrode materials may allow for decreased irreversible capacity loss during cycling in lithium-ion battery systems.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 62/313,554, entitled “Method For Controllable SynthesisOf Carbon Based Battery Electrode Material,” filed on Mar. 25, 2016,which is expressly incorporated by reference herein in its entirety.

BACKGROUND OF THE DISCLOSURE

Lithium-ion secondary cells or batteries are commonly used as powersources in portable electronic devices. Such rechargeable cellsgenerally use a lithium transition metal oxide (e.g., lithiumcolbaltate) positive electrode and a negative electrode composed of ahighly porous carbonaceous material, typically graphite. Thecarbonaceous material, however, may also include other carbons, metaland/or a pyrolyzed organic material. A lithium-ion soluble electrolyteis provided between the two electrodes, and the cell is charged. Duringthe electrochemical process of charging, some of the lithium ions in thepositive electrode migrate from the positive electrode (serving as theanode) and intercalate into the negative electrode (serving as thecathode). The ability of an electrode to accept ions for intercalationdepends largely, for example, on the crystallinity, the microstructure,the porosity, and/or the micromorphology of the material comprised bythe electrode. During discharge, the negative charge held by thenegative electrode (now serving as the anode) is conducted out of thebattery through its negative terminal and the lithium ions migratethrough the electrolyte and back to the positive electrode (now servingas the cathode). While it is understood that the terms “anode” and“cathode” apply to each of the negative and positive electrodesdepending upon whether the cell is being charged or is discharging,hereinafter the term “anode” is used to refer to the negative electrode,and the term “cathode” is used to refer to the positive electrode.

During the first electrochemical intercalation of lithium ions into thecarbonaceous anode material, some lithium is irreversibly consumed and asignificant amount of capacity cannot be recovered in the followingdischarge. This irreversible capacity loss, which mainly depends on thetype of carbonaceous anode material and electrolyte solution used, isexplained on the basis of the reduction of the electrolyte solution andthe formation of a passivating film at the Li_(x)C interface. Chemicalcombination of lithium to the active surface functional groups of carbonmay also play an important role in this irreversible capacity loss.Another source of irreversible capacity is the reduction of Li ionconcentration due to the ions' strong binding with anode materialfollowed by the growth of dendritic forms of Li. This irreversiblecapacity loss affects the cell balancing and lowers the energy densityof lithium-ion batteries.

At present, special-types of “hard carbon” or graphite are used as anodematerials in commercial lithium-ion batteries. The carbon/graphitematerials deliver a reversible specific capacity of only ˜370 mAh/g,corresponding to the chemical formula of LiC₆, as compared to 3830 mAh/gfor metallic lithium. The main advantage of these special carbonmaterials is their relatively low irreversible capacity loss (≦10%)combined with their high storage capacity (>400 mAh/g). However, themethods of synthesizing these special carbon materials do not allowindependent fine tuning or control of pore size distribution,crystallinity and surface area of the materials, which could furtherimprove capacity and reduce irreversible capacity loss.

Based on the foregoing, there is a need in the art to synthesizeinexpensive carbon-based electrode materials that have increasedreversible capacity and decreased irreversible capacity loss for use inlithium-ion battery systems. It would be further advantageous if thematerials could be synthesized using methods that could control poresize distribution, surface area, and crystallinity of the electrodematerial.

BRIEF DESCRIPTION OF THE DISCLOSURE

Aspects of the present disclosure are directed generally to compositematerials for use in, for example, lithium-ion batteries. The compositematerials include graphite particles bridged by at least one pentagonring. These composite materials may be used in electrodes to allow fordecreased irreversible capacity loss during cycling similar to expensive“hard carbon” materials, and may be synthesized to control pore sizedistribution, surface area, and crystallinity of the carbon compositematerials.

In one aspect, the present disclosure is directed to a composite carbonmaterial comprising at least a first graphite particle connected to atleast a second graphite particle, wherein the first and second graphiteparticles are connected by a pentagon carbon ring precursor.

In another aspect, the present disclosure is directed to a carbon-basedelectrode material comprising at least a first graphite particleconnected to at least a second graphite particle, wherein the first andsecond graphite particles are connected by a hemisphere of fullerene.

In another aspect, the present disclosure is directed to a method ofsynthesizing a carbon-based electrode material. The method comprises:mixing at least a first graphite particle and a second graphite particlewith at least one hemisphere of fullerene; and heating the mixture to atemperature of up to 2000° C. under presence of a hydrocarbon gas.

In another aspect, the present disclosure is directed to a method ofmaking a composite material. The method includes providing a mixture ofat least a first graphite particle and a second graphite particle withat least one pentagon ring precursor; and treating the mixture to bridgethe first graphite particle and the second graphite particle with atleast one pentagon ring.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic depicting a method for controllably synthesizingthe carbon-based electrode material of one aspect of the presentdisclosure.

FIG. 2A shows an example result of Brunauer-Emmett-Teller (BET) surfacearea measurements corresponding to an example hard carbon materialaccording to the present disclosure.

FIG. 2B shows an example result of Brunauer-Emmett-Teller (BET) surfacearea measurements corresponding to an example hard carbon materialaccording to the present disclosure.

FIG. 2C shows an example result of Brunauer-Emmett-Teller (BET) surfacearea measurements corresponding to an example hard carbon materialaccording to the present disclosure.

FIG. 3A shows mercury porosity measurements corresponding to an examplehard carbon material according to the present disclosure.

FIG. 3B shows mercury porosity measurements corresponding to an examplehard carbon material according to the present disclosure.

Table 1 shows example results of BET surface area measurementscorresponding to an example hard carbon material according to thepresent disclosure and as shown in FIGS. 2A-2C.

DETAILED DESCRIPTION OF THE DISCLOSURE

Generally, the present disclosure is directed to composite materials foruse in, for example, lithium-ion batteries and to methods forsynthesizing the composite materials. The materials are synthesized toinclude graphite particles bridged by one or more pentagon carbon rings(such as by hemispheres of fullerene). Electrodes made from thesematerials may allow for decreased irreversible capacity loss duringcycling. As used herein, the term “electrode” generally refers to anelectrical conductor. For example, in one illustrative example, an“electrode” may refer to an anode. Further, the methods of synthesizingthese materials may allow for controlled pore size distribution, surfacearea, and crystallinity of the materials.

The composite materials of the present disclosure include graphiteparticles bridged by a pentagon carbon ring (e.g., hemispheres offullerene). As used herein, “bridge,” “bridged,” or “bridging” refers toconnecting at least one graphite particle to at least a second graphiteparticle by a pentagon ring (or material containing a pentagon ring). Ina non-limiting example, the pentagon ring may be carbon based, such aspart of a hemisphere of fullerene. Typically, more than two graphiteparticles are connected using one or more pentagon rings, therebycreating a mesh network-type carbon-based electrode material as shown inFIG. 1.

Generally, graphite is a suitable carbonaceous material for use informing an electrode (e.g., an anode) in a lithium-ion battery becauseof its ability to provide an initial high reversible capacity for thebattery. As used herein, “graphite” refers to carbonaceous materialswith a layered structure, typically comprising layers of graphene.Examples of graphite material for use in the carbon-based electrodematerials of the present disclosure include, but are not limited to,graphite powder, such as artificial graphite and natural graphite, and apurified product thereof, a graphitized product of electroconductivecarbon black, such as acetylene black and Ketjen black, and carbonfibers, such as vapor phase growth carbon fibers.

Typically, the graphite is in particle or powder form, having an averageparticle diameter of about 1 μm or more, and optionally, about 5 μm ormore, and optionally, from about 1 μm to about 45 μm, and optionally,from about 2.5 μm to about 35 82 m, and optionally from about 5 μm toabout 25 μm. Without being limited to any particular theory, it appearsthat where the average particle diameter is too small, the specificsurface area of the graphite is increased, whereby the irreversiblecapacity is increased to lower the battery capacity. In the case wherethe average particle diameter is too large, conversely, it appears thatthe thickness of the electrode material is restricted, whereby it isdifficult to form a uniform electrode material.

The specific surface area of the graphite is generally about 0.1 m²/g ormore, suitably, optionally about 0.3 m²/g or more, and optionally, about0.5 m²/g or more. In particular aspects, the surface area of thegraphite ranges from about 0.1 m²/g to about 30 m²/g, including fromabout 0.3 m²/g to about 20 m²/g, and including from about 0.5 m²/g toabout 10 m²/g. In the case where the specific surface area is too small,it appears that the rate characteristics of the battery aredeteriorated. In the case where the specific surface area is too large,it appears that the initial efficiency of the battery is too low. Themeasurement of the specific surface area may be attained by the BETmethod.

The carbon-based electrode material typically includes graphite. Inaddition to graphite, the carbon-based electrode material of the presentdisclosure may include one or more pentagon carbon rings made from oneor more pentagon carbon ring precursors. As used herein, “pentagoncarbon ring precursor” interchangeably refers to molecules that includeor are capable of forming one or more C5 pentagon rings. The pentagonstructures of these precursors may improve the graphite particlesconnection and rigidity.

In one aspect, the pentagon carbon ring precursor is fullerene or afracture thereof. As used herein, “fullerene” refers to any productmaterials that are formed utilizing a fullerene generating process, andis generally carbon material in a spherical shell form. Examplefullerene generating processes include, without limitation,high-intensity laser desorption, arc discharge (e.g., aKratschmer-Huffman process), and combustion flame generation.

Example fullerene for use in the carbon-based electrode materialsdescribed herein include, for example, C₆₀, C₇₀, C₇₄, C₇₈, C₈₀, C₈₂,C₈₄, C₈₆, C₈₈, C₉₀, C₉₂, C₉₄, C₉₈, C₁₀₀₋₂₅₀, and C₂₅₀₊ (e.g., C₂₇₀), aswell as dimers and trimers of these compounds, and as well as fractionsthereof (e.g., fractions of C₆₀, C₇₀). Combinations of these fullerenecompounds may also be used without departing from the presentdisclosure. For example, C₆₀, C₇₀ and a dimer and a trimer of thesecompounds are suitable, since they can be easily obtained industriallyand have a high affinity to the surface of graphite.

As shown in FIG. 1, it is suitable for the fullerene compound (1) to besplit in half to form a hemisphere (2) of fullerene.

When used in lithium-ion batteries, the carbon-based electrode materialcan further include binder (e.g., polymeric binders such aspolyvinylidene fluoride, hexafluoropropylene, polyethylene, polyethyleneoxide, polypropylene, polytetrafluoroethylene, polyacrylates, etc.), andother additives, such as an electroconductive agent, and others as knownin the lithium-ion battery art. The species and contents of thematerials may be appropriately adjusted depending on the batteryperformance demanded.

The present disclosure is further directed to methods of synthesizingthe carbon-based electrode materials. Generally, the methods includeproviding a mixture of the graphite particles (3) and the pentagon ringprecursor in a reactor under inert gas flow; and heating the mixture forexample, up to a temperature of up to 2000° C., under presence of ahydrocarbon gas. According to some aspects, the pentagon ring precursormay comprise one or more hemispheres (2) of fullerene.

According to some aspects, the hemispheres of fullerene may be preparedby opening a precursor (for example, a fullerene sphere) using thermaloxidation. According to some aspects, fullerendione (derived frompristine C₆₀) may be used as a precursor. For example, the precursor inmedia (for example, toluene, acetone, ethanol, methanol, and/or mixturethereof) may be deposited onto a substrate (for example, printed onto aST cut quartz substrate by poly(dimethylsiloxane) (PDMS) stamp or placedonto the substrate via pipet), and then the media may be removed byevaporation in air followed by baking (for example, at 150° C.).According to some aspects, the precursor may then undergo thermaloxidation (for example, in air). For example, according to some aspects,the precursor may be heated to a temperature of about 300-500° C. for 30minutes in a 1.8 cm tube furnace, and then heated to a temperature ofabout 900° C. The sample may then be treated with water to removeamorphous carbon, and then annealed (for example, at about 900° C. for 3minutes) to remove carboxyl groups at the open end of the hemisphere,thereby activating the hemisphere.

According to some aspects, the hemispheres of fullerene may be preparedby opening a precursor wherein the precursor comprises C₆₀ fullerenes.For example, the hemispheres of fullerene may be prepared by dispersingC₆₀ fullerenes in media (for example, toluene, acetone, ethanol,methanol, and/or mixture thereof), depositing the C₆₀ fullerenes on asubstrate (for example, an ST-cut quartz substrate), and then usingpretreatment steps to open and/or functionalize (i.e., activate) thefullerenes. According to some aspects, the pretreatment steps maycomprise applying an oxidation treatment (for example, heating thedeposited fullerenes in air at a temperature of about 500° C. for about75 minutes) followed by a short (for example, 2 minutes) exposure to H₂Oand a brief (for example, 3 minutes) exposure to H₂ in order tofunctionalize dangling bonds at the open ends of the resultanthemispheres. It will be appreciate that each method step may beoptimized, for example, by optimizing the environment and/or time.

According to some aspects, hemispheres of fullerene may be prepared bypartially dissolving a precursor (for example, C₆₀ fullerenes). Forexample, the fullerenes may be heated in the presence of a carboncontaining gas to a temperature below that at which the fullerenessublime (for example, between about 500° C. to 700° C. at atmosphericpressure) such that the fullerenes may partially dissolve intohemispheres. According to some aspects, the fullerenes may be dissolvedin media (for example, toluene, acetone, ethanol, methanol, and/ormixture thereof) and deposited on a substrate or catalyst (for example,a metal catalyst) prior to or after heating.

To thoroughly mix the graphite particles (3) and fullerene hemispheres(2), the graphite and fullerene are placed into the reactor with one ormore inert gas and mixed for a period sufficient to mix the graphiteparticles and fullerene hemispheres.

Example inert gases for use in the processing chamber include argon,helium, nitrogen, mixtures thereof, and any other inert gases or gasmixtures known in the art.

In one particularly suitable aspect, the mixture is heated to atemperature of from greater than 1500° C. to about 2000° C.

Example hydrocarbon gases include methane, ethylene, acetylene, ethanol,benzene, methanol, carbon-based polymer, a nano-carbon material,mixtures thereof, and/or any other gases or gas mixtures known in theart.

Using the methods described above, graphite particles and fullerenehemispheres are bound together such that the graphite particles arebridged by the fullerene hemispheres to form a composite materialcomprising a plurality of pores (4). Varying the ratios of graphite andfullerene hemispheres, varying the numbers of carbon rings between thegraphite particles, varying the graphite aspect ratio (i.e., the ratioof a lateral dimension of the graphite to the thickness of thegraphite), and/or by varying the heating temperatures used in themethods described herein, the porosity, surface area and/orcrystallinity can be varied of the resulting carbon-based electrodematerial. According to some aspects of the present disclosure, theresulting composite material may comprise a plurality of pores (4) thatare capable of reacting interstitially with ions, for example, lithiumions, thereby providing a material capable of acting as an electrode(e.g., an anode) in an electrochemical battery. For example, in the caseof lithium-ion batteries, the material may comprise pores capable oftaking up and releasing lithium ions (i.e., lithium ion insertion andextraction) through intercalation or a similar process. As used herein,the term “pore” refers to an opening or depression in the surface, or atunnel, in the material, for example, between graphite particles and/orcarbon rings. According to some aspects, the pore size of the pluralityof pores may be varied by any of the means described herein. Forexample, by increasing the ratio of carbon rings to graphite, the poresize may be increased.

According to some aspects, the pores may have a pore size of from abouta few nanometers up to hundreds of micrometers. For example, the poresmay have a pore diameter in the range of about 0.001 to 300 nm,preferably in the range of about 0.01 to 200 nm, and more preferably inthe range of about 0.1 to 150 nm. According to some aspects, the poresmay have an average pore diameter of from about 0.1 to 20 nm, preferablyfrom about 0.1 to 15 nm, and even more preferably from about 0.1 to 10nm. According to some aspects, the pores may have an average porediameter of from about 0.1 to 50 nm, preferably from about 10 to 40 nm,and even more preferably from about 20 to 30 nm.

According to some aspects, the pores may have a pore diameter in therange of about 0.0001 to 50 μm, preferably in the range of about 0.0001to 10 μm, and more preferably in the range of about 0.0001 μm to 5 μm.According to some aspects, the pores may have an average pore diameterfrom about 0.1 to 20 μm, preferably from about 0.1 to 10 μm, morepreferably from about 0.1 μm to 7 μm, and even more preferably fromabout 0.5 μm to 4 μm. According to some aspects, the pores may have anaverage pore diameter from about 0.1 to 50 nm, preferably from about 0.1to 40 nm, more preferably from about 0.1 to 30 nm, even more preferablyfrom about 0.1 to 20 nm, and most preferably from about 1 to 10 nm. In anon-limiting example, the average pore diameter is based on aBrunauer-Emmett-Teller (BET) measurement.

According to some aspects, the pores may have a pore volume of fromabout 10 ⁻²⁴ to 10 ⁻⁶ liters.

According to some aspects, the volume of the pores in the material maybe in the range of about 0.00001 to 0.00040 cm³/g, preferably in therange of about 0.00001 to 0.00030 cm³/g, and more preferably in therange of about 0.00002 to 0.00020 cm³/g. According to some aspects, theaverage volume of the pores in the material may be from about 0.0001 toabout 1.0 cm³/g, preferably from about 0.0001 to 0.1 cm³/g, morepreferably from about 0.0001 to about 0.01 cm³/g, and even morepreferably from about 0.001 to about 0.01 cm ³/g.

According to some aspects, the material may have a specific surface areaof from about less than 1 m²/g to more than 100 m²/g. For example,according to some aspects, the material may have a specific surface areaof from about 0.01 to 20 m²/g, preferably from about 0.1 to 15 m²/g,more preferably from about 1.0 to 10 m²/g, and even more preferably fromabout 1.0 to 6.0 m²/g.

According to some aspects, the material may have a density of from about1 to 103 kg/m³. According to some aspects, the pore size distributionmay range from micro to meso to macro, and may be either monomodal,bimodal, or multimodal (i.e., may comprise one or more differentdistribution of pore sizes). According to some aspects, the material mayhave a pore distribution from nanometers up to millimeters. According tosome aspects, the pores may have a pore length from a few nanometers upto several centimeters.

According to some aspects, the material may comprise a pore size, porevolume, surface area, density, pore size distribution, and/or porelength similar to a hard carbon material.

Table 1 and FIGS. 2A-2C show exemplary results of Brunauer-Emmett-Teller(BET) surface area measurements corresponding to an example porous hardcarbon material according to aspects of the present disclosure.Specifically, Table 1 and FIGS. 2A-2D correspond to a hard carbonmaterial that is similar to the hard carbon material disclosed in U.S.Patent Application Publication No. 2007/0287068, that is, an electrode(such as an anode) formed from a pitch-based hard carbon having anaverage particle size of preferably between 5 and 15 μm, a surface areaof between 0.5 and 15 m²/g, an interlayer spacing d₀₀₂ of between 0.355and 0.400 nm, and a density of between 1.50 and 1.60 g/cm³. Theelectrode of hard carbon material may be formed by mixing the hardcarbon with polyvinylidene to form a paste together withN-methyl-2-pyrrolidore, which may then applied to a copper foil, dried,and then pressed to provide the electrode.

It should thus be understood that Table 1 and FIGS. 2A-2C showmeasurements that are similar to or the same as measurementscorresponding to the material of the present disclosure, as the materialof the present disclosure may comprise a pore size, pore volume, surfacearea, density, pore size distribution, and/or pore length similar tothat of the hard carbon material of Table 1 and FIGS. 2A-2C.

As can be seen, for example, in Table 1, the adsorption average porediameter was found to be about 6.76 nm, and the Barrett-Joyner-Halenda(BJH) adsorption average pore diameter was found to be about 27.95 nm orabout 25.56 nm. FIGS. 3A and 3B show mercury porosity measurementscorresponding to an example hard carbon material according to thepresent disclosure. As can be seen by these figures, the hard carbonmaterial showed a pore size distribution with a peak ranging from about0.5 to 4 μm pore diameter, with multiple peaks ranging from about 0.1 to20 nm.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any systemsand performing any incorporated methods. The patentable scope of thepresent disclosure is defined by the claims, and may include otherexamples that occur to those skilled in the art. Such other examples areintended to be within the scope of the claims if they have structuralelements that do not differ from the literal language of the claims, orif they include equivalent structural elements with insubstantialdifferences from the literal languages of the claims.

1. A carbon-based electrode material comprising at least a firstgraphite particle connected to at least a second graphite particle,wherein the first and second graphite particles are connected by amaterial including a pentagon carbon ring.
 2. The carbon-based electrodematerial as set forth in claim 1 wherein at least one of the first orsecond graphite particles have an average particle diameter of fromabout 1 μm to about 45 μm.
 3. The carbon-based electrode material as setforth in claim 1 wherein at least one of the first or second graphiteparticles have a specific surface area of from about 0.1 m²/g to about30 m²/g.
 4. The carbon-based electrode material as set forth in claim 1wherein the material including a pentagon carbon ring is selected fromthe group consisting of fullerene and fractions of fullerene.
 5. Thecarbon-based electrode material as set forth in claim 1, wherein thematerial comprises a plurality of pores capable of reactinginterstitially with lithium ions.
 6. The carbon-based electrode materialas set forth in claim 5, wherein the pores are defined by a plurality ofgraphite particles bridged by fullerene hemispheres and the carbon-basedelectrode material has a pore size distribution that is the same as thatof a hard carbon material.
 7. The carbon-based electrode material as setforth in claim 5, wherein the plurality of pores have an averagediameter of from about 0.1 to 40 nm.
 8. The carbon-based electrodematerial as set forth in claim 7, wherein the plurality of pores have anaverage diameter of from about 1 to 10 nm.
 9. A carbon-based electrodematerial comprising at least a first graphite particle connected to atleast a second graphite particle, wherein the first and second graphiteparticles are connected by a hemisphere of fullerene.
 10. Thecarbon-based electrode material as set forth in claim 9 wherein at leastone of the first or second graphite particles have an average particlediameter of from about 1 μm to about 45 μm.
 11. The carbon-basedelectrode material as set forth in claim 9 wherein at least one of thefirst or second graphite particles particular have a specific surfacearea of from about 0.1 m²/g to about 30 m²/g.
 12. The carbon-basedelectrode material as set forth in claim 9 wherein the fullerenecomprises at least one of C₆₀ fullerene, C₇₀ fullerene, C₇₄ fullerene,C₇₈ fullerene, C₈₀ fullerene, C₈₂ fullerene, C₈₄ fullerene, C₈₆fullerene, C₈₈ fullerene, C₉₀ fullerene, C₉₂ fullerene, C₉₄ fullerene,C₉₈ fullerene, C₁₀₀₋₂₅₀ fullerene, C₂₅₀₊ fullerene, a dimer thereof, anda trimer thereof.
 13. The carbon-based electrode material as set forthin claim 9 wherein the fullerene comprises a mixture of C_(α)fullereneand C₇₀ fullerene.
 14. The carbon-based electrode material as set forthin claim 9, wherein the material comprises a plurality of pores capableof reacting interstitially with lithium ions.
 15. The carbon-basedelectrode material as set forth in claim 14, wherein the plurality ofpores have an average diameter of from about 0.1 to 40 nm.
 16. Thecarbon-based electrode material as set forth in claim 15, wherein theplurality of pores have an average diameter of from about 1 to 10 nm.17. A method of synthesizing a carbon-based material, the methodcomprising: treating a mixture of graphite particles and fullerenehemispheres to form a composite material comprising a plurality of poresdefined by graphite particles bridged by the hemispheres of fullerene.18. The method as set forth in claim 17, wherein the graphite particlesand hemispheres of fullerene are mixed under inert gas flow.
 19. Themethod as set forth in claim 18, wherein the inert gas is argon.
 20. Themethod as set forth in claim 17, wherein the treating step comprisesheating the mixture to a temperature of from about 1500° C. to about2000° C.
 21. The method as set forth in claim 20, wherein the treatingstep further comprises heating the mixture in the presence of ahydrocarbon gas selected from the group consisting of methane andpropane.
 22. A method of making a composite material, the methodcomprising: providing a mixture of at least a first graphite particleand a second graphite particle with at least one pentagon ringprecursor; and treating the mixture to bridge the first graphiteparticle and the second graphite particle with at least one pentagonring.
 23. The method of 22, wherein the pentagon ring precursor is acarbon-based pentagon ring precursor.
 24. The method of 23, whereintreating the mixture includes heating the mixture in the presence of atleast one hydrocarbon gas.